The prayer book is a reward from the The Great Brain Robbery quest. By right clicking on it while wearing the Holy symbol, the player can use Prayer to heal poison at the cost of some prayer points (see last paragraph). Sometimes the poison will only be partially healed if you run out of prayer points.
The Prayer book should ideally be used after combat because the monster a player is fighting could poison again, causing the player to drain his/her prayer points further (to heal poison), and wasting time in which the monster could have been attacked. An alternative to the prayer book use is a Super anti-poison potion.
If lost, the Prayer book can be reclaimed in the Monastery where it was originally found or in a player-owned house bookcase. In the bookcase it is called "Prayer of Deliverance of Poisons." Although this book has a 'destroy' option, it is possible to get more than one copy. This can be done by keeping the book in your bank while you obtain a new one from your player-owned house bookcase.
A good place to use the Prayer book is on Ape Atoll in the Gorilla temple, where there is a prayer altar surrounded by level 167 Gorillas and level 1 poisonous spiders. Using the Prayer Book to counter the poison, fighting the Gorillas after a while will allow a player to fight the creatures without them automatically attacking. During the fight, protect from melee prayer is used sometimes in conjunction with chivalry or piety prayers.
To calculate how many prayer points it will cost, subtract the poison damage by 2. For example, if someone is poisoned for 8 damage each time, it will take 6 prayer points to heal him or her. If a player only has a portion of the required prayer points needed to cure poison, he or she will only be partially healed of poison and the remaining few points will be drained.
Being the full and proper prayer against morbidities of the flesh caused by venoms and potions of both a natural and mystic origin. Take up your blessed symbol of the Saradomin, cast in silver. Hang it about thy neck, where all may see it and thus know of your devotion. Pray in a loud, clear voice to the Saradomin for relief from that which ails you. Appropriate calls for aid should be accompanied by a sacrifice of prayer energy, with deadlier venoms requiring a greater sacrifice. Should the supplicant lack piety, they will not be cured. Should they possess a measure of piety equal or even greater than that which assails them, the poison will reduce or even cure, while a like amount of Prayer energy will be taken to do this. Be wary: should the supplicant only partially remove the venom thought the application of prayer, their body must still bear the brunt of its wickedness. Also...(The rest of the book seem to have been vandalised, charred and slightly bled upon.)